Oversubscribing Atm Ports; Minimizing Latency On The Sar Scheduler; Hrr Scheduler Behavior And Strict-Priority Scheduling; Zero-Weight Queues - Juniper JUNOSE 11.1.X - QUALITY OF SERVICE CONFIGURATION GUIDE 3-21-2010 Configuration Manual

JUNOSe 11.1.x Quality of Service Configuration Guide

Oversubscribing ATM Ports

You cannot oversubscribe ATM ports and still achieve low latency with relative
strict-priority scheduling. There are several ways to ensure that ports are not
oversubscribed. The most common is to use a per-VC scheduler by configuring the
HRR scheduler with either ATM VP or VC node shaping (using the atm-vp node or
atm-vc node commands), and setting the sum of the shaping rates less than the port
rate. In these scenarios, the cell residency in the SAR scheduler is minimal, and cell
scheduling does not interfere with relative strict priority.

Minimizing Latency on the SAR Scheduler

There are two methods you can use to control latency on the SAR scheduler. In the
first method, you set the ATM QoS port mode to low-latency mode. In low-latency
mode, the HRR scheduler controls scheduling, buffering in the SAR scheduler is
limited, and latency caused by the SAR scheduler is minimized.
You can also use the default no qos-mode-port mode of SAR operation to minimize
the latency induced by the SAR. In this method, you set qos shaping-mode cell and
shape an OC-3 ATM port to 149 Mbps, or an OC-12 ATM port to 600 Mbps. By
throttling the rate at which the HRR scheduler delivers packets to the SAR, you bound
SAR buffering and latency. This approach retains the flexibility to configure different
ATM QoS in the SAR, including shaped VP tunnels, UBR+PCR, nrtVBR, and CBR
services.
To set the SAR mode, use the qos-mode-port command. For more information about
operational modes on ATM interfaces, see "ATM Integrated Scheduler Overview" on
page 159.
NOTE: Controlling latency is not normally required. If you undersubscribe the port
rate in the HRR scheduler, you can obtain latency bounds without modifying the
SAR mode of operation.

HRR Scheduler Behavior and Strict-Priority Scheduling

The HRR scheduler does not offer native strict-priority scheduling above the first
scheduler level in the hardware; however, you can configure very large weights in
the round robin in the HRR scheduler to obtain approximate strict-priority scheduling.
Note that under conditions of low VC bandwidth and large packet sizes, latency and
jitter increase because of the inherent propagation delay of large packets over a small
shaping rate. The following sections describe additional configuration steps that will
ensure that no more than a single nonstrict packet can precede a strict-priority packet
on the VC.

Zero-Weight Queues

To reduce latency and jitter, you can configure the relative strict-priority queue with
a weight of 0 (zero), which gives the queue a weight of 4080. When a packet arrives
at a zero-weighted queue, the queue remains in the active WRR until it is exhausted,
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Comparison of True Strict Priority with Relative Strict Priority Scheduling